This invention relates to a magnetoresistive displacement sensing transducer cooperating with a magnetic grating to sense relative linear or rotational displacement. More particularly this invention relates to apparatus for sensing relative displacement of a magnetic grating using sensing elements consisting of anisotropic magnetoresistive stripes and to signal processing apparatus therefor.
A magnetoresistive sensing transducer is described in U.S. Pat. No. 3,949,345, the disclosure of which is incorporated herein by reference. The reference U.S. patent discloses a magnetoresistive sensing element consisting of first and second sets of parallel strips of anisotropic magnetoresistive material such as, for example, nickel-cobalt or nickel-iron desposited on, or etched from a layer on an insulating substrate. The stripes and their interconnecting conductors are preferably deposited by standard thin-film techniques on a suitable substrate such as glass. All stripes in a sensing element are connected in series between the terminals of a voltage source and an output terminal is provided at the junction of the first and second sets of stripes.
An anisotropic magnetoresistive material has a resistivity (specific resistance) that varies according to the direction of a magnetic field applied thereto. When a magnetic field is perpendicular to current in the stripes, the resistivity thereof is minimum; and when the magnetic field is parallel to current in the stripes, the resistivity thereof is maximum. The resistance of a stripe of uniform thickness is proportional to the length and inversely proportional to the width thereof.
A magnetic grating disposed adjacent the sensing element has alternating north and south poles in the direction of relative displacement. The spacing between corresponding magnetic poles (e.g. from north pole to adjacent north pole) defines the pitch or wavelength .lambda. of the magnetic grating. The spacing between the first and second sets of stripes is established relative to the wavelength of the magnetic grating such that when the first set of stripes exhibits maximum resistivity, the second set of stripes exhibits minimum resistivity and vice versa. The first and second sets of stripes form a voltage divider with an output being taken across one of the sets of stripes. Since the resistivity (and resistance) of the two sets of stripes vary in opposite directions as the relationship between them and the magnetic poles in the magnetic grating varies, the output also varies in proportion to such relationship.
The measurement resolution, or minimum displacement which can be sensed, using the sensing element of the referenced U.S. patent is determined by the minimum usable wavelength .lambda. of the magnetic grating which is, in turn, limited by the finite minimum size of the sets of stripes. In practice, resolution of better than 1 mm cannot be achieved without using relatively expensive phase modulation detection for interpolation.
In the device of the referenced U.S. patent, it is desirable to use a plurality of such sensing elements connected in series disposed in the direction of displacement. Using a plurality of sensing elements in this manner tends to make the device large. When the wavelength of the magnetic grating is, for example, 2 mm, and when the number of sensing elements is, for example, 10, the length of the set of 10 sensing elements is at least 40 mm.
The poor inherent resolution and the large size of the sensing device according to the referenced U.S. patent increases the cost of producing a practical sensing device.
An attempt has been made in Japanese Patent Application No. 114,699/1977 to improve resolution with reduced size by substituting a single one of a set of parallel magnetoresistive stripes for each of the plurality of sets of stripes in the referenced U.S. patent. The parallel stripes are electrically connected in series in zig-zag fashion. Each of the stripes depends on leakage magnetic flux from a single magnetic pole for controlling the resistivity thereof. However, when the wavelength .lambda. of the magnetic grating is reduced to improve the resolution of the device, the leakage magnetic flux is so drastically reduced that the related stripes are not saturated. This leads to problems with magnetic hysteresis.
A partial solution to attain saturation of the stripes with relatively short wavelength .lambda. of a magnetic grating includes depositing a thin film of high magnetic permeability to form closed magnetic paths about pairs of adjacent stripes. The closed magnetic paths induce increased leakage flux from the magnetic grating to thus increase the magnetic field to which the stripes are exposed sufficiently to achieve saturation and to avoid hysteresis effects.
The benefits derived from the use of thin-film closed magnetic paths are limited by the relatively low value of magnetic permeability attainable in a thin film.
A different approach to achieving saturation of the magnetoresistive stripes is described in an article entitled "Non-Contact Switch Is Based on Magnetoresistance", which appeared on page 3E in the May 1, 1975 issue of Electronics Magazine (McGraw-Hill). A bias magnetic field, on the order of 50 Oersteds, is applied to the magnetoresistive stripes to maintain them in the saturated condition. Thus any change in output due to proximity of the magnetic grating is free of interference from hysteresis. The article also notes that disposing the bias magnetic field at an angle of 45 degrees to the stripes reduces the originally small temperature coefficient of magnetoresistance to zero.
A further attempt to improve the resolution of a magnetoelectric transducer employs stripes of magnetoresistive material in a repeating symmetrical triangular wave pattern, having a wavelength equal to the wavelength .lambda. of the magnetic grating, to which a constant bias magnetic field is applied having a direction normal to the magnetic fields of the magnetic grating. Resultant magnetic field vectors are produced by the interaction of the north and south poles of the magnetic grating with the bias magnetic field. The resultant magnetic field vectors of the magnetic grating and the bias magnetic field are parallel to adjacent pairs of stripes at one position, yielding maximum resistivity, and make an angle with the magnetic stripes at another position yielding minimum resistivity. Two triangular wave patterns are simultaneously exposed to the fields of the magnetic grating. The two triangular wave patterns are so disposed that a maximum output signal from one pattern coincides with a minimum output signal from the other pattern and vice versa.
The triangular wave element has the disadvantage that reduced output efficiency results from the fact that the stripes of each adjacent pair are disposed over half the wavelength .lambda. of the magnetic grating. This is believed to cause at least partial cancellation of the magnetoresistance effect.